graphics/drm-51*-kmod: Add Makefile.version

Projects that live outside of drm-kmod (such as nvidia-drm) will need
to query the version of drm-kmod built against in order to grab the
correct sources. This adds a Makefile.version they can include to do
so.

Differential Revision: https://reviews.freebsd.org/D39885

(cherry picked from commit 4eef420a2503f1e573d4925797f9118b9decd1ed)